94 Domestic Science 



may be taken in a jar of such a size that, when the 

 hydrometer is floating in the liquid, the liquid surface 

 is at the top of the jar, a*nd the reading at the general 

 level of the liquid may be taken through the meniscus. 

 This is not very easy to do with accuracy and cannot, 

 of course, be done in the case of an opaque liquid, and 

 the allowance of half a scale -division is sufficiently exact 

 for all practical purposes. Test the accuracy of the 

 graduation of the hydrometer by finding whether, when 

 floated in distilled water, the scale-reading is exactly 

 1*000. A further check may be made by finding the 

 specific gravity of the salt solution by a different 

 method, such as by the use of a density flask, and noting 

 whether the two results agree. 



Exercise for Student. 



Prepare a solution of salt with a specific gravity of I'OIO by 

 means of the hydrometer. 



56. The lactometer is simply a form of hydro- 

 meter in which the scale has a range which makes 

 the instrument suitable for finding the specific gravity 

 of milk. A common range is from T015 to T040, and 

 the markings on the scale usually consist of the last 

 two figures only of the specific gravity, the TO being 

 understood as to be added to all readings taken with 

 the instrument. Thus a reading of 32' 5 on the scale 

 would show a milk of specific gravity T0325. The 

 value of the information obtained by means of the 

 lactometer in judging of the genuine character of the 

 sample of milk tested will be treated fully in Part II. 



57. In the statement of the Principle of Archi- 

 medes already given, the case of a body immersed 

 in a liquid alone receives recognition. It has been 

 previously mentioned that all fluids both liquid and 

 gaseous exert pressure on bodies in contact with them. 



